Amorphous silicon photovoltaic device having two-layer transparent electrode

ABSTRACT

An amorphous silicon semiconductor of the general formula: a-Si.sub.(1-x-y) C x  N y  containing hydrogen and/or fluorine, which provides an amorphous silicon PIN junction photovoltaic device having an improved conversion efficiency when it is used as a P-type or N-type layer on the light impinging side of the PIN junction photovoltaic device. Also, the conversion efficiency of an amorphous silicon PIN junction photovoltaic device is improved by using a two-layer film structure of ITO and SnO 2  as a transparent electrode for the photovoltaic device, with the SnO 2  layer contacting the P or N layer. The improvement is particularly marked in the case of heterojunction photovoltaic devices.

BACKGROUND OF THE INVENTION

The present invention relates to an amorphous silicon semiconductor andan amorphous silicon photovoltaic device.

Since it was found in 1976 by W. E. Spear et al. that the conductivityof the amorphous silicon obtained by a plasma decomposition method ofsilane (SiH₄) could be greatly altered by doping with phosphine (PH₃)and diborane (B₂ H₆) and an amorphous silicon solar cell was fabricatedon an experimental basis in 1976 by D. E. Carlson et al., the amorphoussilicon solar cell has attracted attention and many studies with the aimof improvement of its efficiency have been made.

The Schottky barrier type, PIN type, MIS type and heterojunction typeare known structures of photovoltaic devices using an amorphous siliconthin film. The former three types promise to provide highly efficientsolar cells. For instance, a Schottky barrier type solar cell made by D.E. Carlson et al. in 1977 showed a conversion efficiency of 5.5%, an MIStype solar cell made by J. I. B. Wilson et al. in 1978 showed aconversion efficiency of 4.8%, and a PIN junction solar cell made byYoshihiro Hamakawa in 1978 showed a conversion efficiency of 4.5%.

In the case of the PIN junction solar cell, the P or N type amorphoussilicon has a short carrier life time and, therefore, fails to provideeffective carriers. Also, the P-layer suffers from light absorption lossbecause it has a higher light absorption coefficient than the I-layer.In order to eliminate these drawbacks, an inverted PIN junctionphotovoltaic device has been proposed. This photovoltaic device isconstructed so that the light impinges on the N-type amorphous siliconside. Since this device has a smaller light absorption coefficient thanthe P-type, it is believed to be more advantageous, though slightly.Nevertheless this N-type amorphous silicon is no better than the P-typein the sense that it similarly suffers from light absorption loss.

Also, in the case of the PIN junction solar cell, it is necessary toform a transparent electrode on the light-impinging side, and ITO(indium-tin-oxide) (In₂ O₃ +SnO₂) or SnO₂ has been employed as atransparent electrode. However, cells with the ITO electrode have thedisadvantage that while the fill factor is good, the open circuitvoltage is low, and cells with the SnO₂ electrode have the disadvantagethat while the open circuit voltage is high, the fill factor is bad.

It is an object of the present invention to provide an amorphous siliconsemiconductor suitable for use in fabricating an amorphous silicon PINjunction photovoltaic device.

Another object of the invention is to provide an amorphous silicon PINjunction photovoltaic device having an improved conversion efficiency.

It is a still further object to provide a two-layer transparentelectrode on the light impinging side of an amorphous silicon type solarcell.

These and other objects of the present invention will become apparentfrom the description hereinafter.

SUMMARY OF THE INVENTION

It has now been found that the short-circuit current density and theopen circuit voltage are greatly improved by using in at least one ofthe P and N layers of a PIN junction photovoltaic device a P-type orN-type doped thin film of a hydrogenated amorphous silicon carbonnitride of the general formula: a-Si.sub.(1-x-y) C_(x) N_(y) :H or apartially fluorinated amorphous silicon carbon nitride of the generalformula: a-Si.sub.(1-x-y) C_(x) N_(y) :F:H (they being hereinafterreferred to merely as "amorphous silicon carbon nitride" or"a-Si.sub.(1-x-y) C_(x) N_(y) ").

In accordance with the present invention, there is provided an amorphoussemiconductor having the general formula: a-Si.sub.(1-x-y) C_(x) N_(y).

The present invention also provides an improved amorphous silicon PINjunction photovoltaic device in which at least one of the P-type andN-type amorphous silicon semiconductors thereof is an amorphoussemiconductor of the general formula: a-Si.sub.(1-x-y) C_(x) N_(y).

In one preferred embodiment, this invention provides in an amorphoussilicon derivatives PIN junction photovoltaic device, the improvementcomprising a two layer transparent electrode consisting of ITO and SnO₂layers that are provided on the P or N layer located on the lightimpinging side so that the SnO₂ layer is in contact with said P or Nlayer, said SnO₂ layer having a thickness of about 30 to about 500angstroms.

The photovoltaic device of the present invention is useful as solarcells, photoswitches, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a schematic view illustrating the structure of aphotovoltaic device of the type wherein the light impinges on theP-layer side;

FIG. 1(b) is a schematic view illustrating the structure of aphotovoltaic device of the type wherein the light impinges on theN-layer side;

FIG. 2 is a diagram showing an energy band profile of the PINheterojunction photovoltaic device of the present invention;

FIG. 3 is a graph showing the relationship between the diffusionpotential (Vd) and the open circuit voltage (Voc) as obtained with theP-type amorphous semiconductor on the window side;

FIG. 4(a) is a schematic view illustrating the structure of aphotovoltaic device of another type according to the present inventionwherein the light impinges on the P-layer side; and

FIG. 4(b) is a schematic view illustrating the according to the presentinvention wherein the light impinges on the N-layer side.

DETAILED DESCRIPTION

The term "amorphous silicon" as used herein means the hydrogenated orfluorinated silicon, and comprehends amorphous silicon derivatives suchasa-SiGe:H, a-SiGe:F:H, a-SiSn:H, a-SiSn:F:H, a-SiSnGe:H, a-SiSnGe:F:Hand soon.

Amorphous silicon used in the present invention is obtained bysubjecting amixed gas containing a silicon compound, e.g. monosilane, apolysilane suchas disilane, silane fluoride or a derivative thereof, ora mixture thereof,and hydrogen or an inert gas such as argon or heliumdiluted with hydrogen,and if necessary, further with GeH₄ and/or SnH₄,to radio-frequency glow decomposition or DC glow discharge decompositionby the capacitive or inductive coupling method. The concentration ofsilane in the mixed gas generally is in the range of from 0.5 to 100%.The concentration of the silane derivatives is suitably determined inaccordance with the composition of the desired product and the glowdischarge decomposition conditions.

The substrate should have a working temperature in the range of fromabout 200° to about 300° C. The substrates used in the invention includea glass sheet having a transparent electrode (e.g. ITO and SnO₂) vacuumdeposited thereon, a metallized polymer film, a metal sheet and allother known materials used as substrates in the fabrication of solarcells.

Typical examples of the basic construction of a solar cell are shown inFIGS. 1(a) and 1(b). Illustrated in FIG. 1(a) is a solar cell of thetype wherein the light impinges on the P-layer side. For instance, thesolar cell of this type has the construction: glass-transparentelectrode-P-I-N-Al. Illustrated in FIG. 1(b) is a solar cell of the typewherein the light impinges on the N-layer side. For instance, the solarcell of this type has the construction: stainlesssteel-P-I-N-transparent electrode. Optionally, other constructions maybe formed by interposing a thin insulation layer or a thin metal layerbetween the P-layer or N-layerand the transparent electrode. Anyconstruction suffices so far as the PIN junction is included as thebasic component.

Intrinsic amorphous silicon (hereinafter referred to as "I-type a-Si")obtained by the glow discharge decomposition of silane or a derivativethereof, silane fluoride or a derivative thereof, or a mixture thereof,and an amorphous silicon derivative such as a-SiGe or a-SiSn, which hasa carrier life time of not less than about 10⁻⁸ second, a density oflocalized states of not more than about 10¹⁸ cm³ eV¹ and a mobility ofnot less than 10⁻⁴ cm² /V·sec., is employedin the present invention asthe I-type layer of amorphous silicon derivative photovoltaic cells.I-type a-Si and the amorphous silicon derivatives of the invention maybe employed alone or in combination thereof as an I-layer. The PINjunction structure is formed by joining P-type and N-type dopedamorphous silicon semiconductors to the I-layer. The constructioncontemplated by the present invention is characterized byusing, in atleast one of the P-layer and N-layer, specifically in the layer on whichthe light impinges, an amorphous semiconductor of the general formula:a-Si.sub.(1-x-y) C_(x) N_(y), containing hydrogen or fluorine. Both ofthe P and N layers may be made of this particular amorphoussemiconductor. The doped layer which does not use the above-mentionedparticular amorphous semiconductor of the present invention is formed bydoping the above-mentioned I-type a-Si with an element of Group III ofthe Periodic Table to provide the P-type a-Si, or by doping with anelement of Group V of the Periodic Table to provide the N-type a-Si.This P-type or N-type a-Si may be used in combination with orreplacedwith the P-type or N-type amorphous silicon derivatives.

The amorphous semiconductor of the present invention is made ofamorphous silicon carbon nitride represented by the general formula:a-Si.sub.(1-x-y) C_(x) N_(y). The amorphous silicon semiconductor of thegeneral formula: a-Si.sub.(1-x-y) C_(x) N_(y) in which x and y satisfythe following equations: 0.05≦x≦0.75, 0.05≦y≦0.75 and 0.05≦x+y≦0.80, ispreferable. The amorphous silicon carbon nitride is obtained bysubjecting the above-mentioned silicon compound, e.g. silicon hydridessuch as SiH₄ or silicon fluorides such as SiF₄, a carbon compound suchas hydrocarbons or carbon fluoride, and a nitrogen compound, e.g.nitrogen orhydrogenated nitrogens such as NH₃ and hydrazine, to the glowdischarge decomposition. It contains 0.5 to 30 atom % of hydrogen and/orfluorine. When the amorphous silicon carbon nitride semiconductor isemployed as a P-layer or an N-layer of a photovoltaic device, it isdoped with P-type or N-type impurities. A P-type or N-type amorphoussemiconductor having an optical band gap of not less than about 1.85 eV,an electric conductivity of not less than about 10⁻⁸ (Ω.cm)⁻¹ at 20° C.and a diffusion potential (Vd) (exhibited in the PIN junction) of notless than about 1.1 volts is preferred.

The amorphous silicon carbon nitride semiconductor of the presentinventionhas a large optical band gap and exhibits a very high opencircuit voltage (Voc), though the semiconductor, when used as a windowmaterial for the PIN junction photovoltaic device, naturally has apossibility of increasing the short-circuit current density (Jsc). Ithas been found thatin the photovoltaic device of the present invention,there is a correlationbetween the diffusion potential (Vd) and the opencircuit voltage of the device as depicted by the band profile of FIG. 2.Although the diffusion potential (Vd) in the case of the inventionexceeds about 1.1 volts, the trend of the relation is nearly constantwithout reference to the kind of the amorphous semiconductor to be usedon the side exposed to the incidentlight. This diffusion potential isthe difference obtained by subtracting the Fermi level (Ef) of the P, Ndoped layers from the optical band gap (Eg.opt) of the amorphoussemiconductor on the side exposed to the light. Let Ecn stand for theenergy level of the conduction band on the N side and Evp for the energylevel of the valence band on the P side, and the activation energies ΔEpand ΔEn can be determined based on thedependency of electricconductivity on temperature, as shown in FIG. 2. Since ΔEp=Ef-Evp holdsfor the P type and ΔEn=Ecn-Ef for the Ntype, there ensueseVd=Eg.opt-(ΔEp+ΔEn). In the case light incidence on the N side, thediffusion potential is similarly obtained by subtracting the Fermi level(Ef) of the P, N layers from the optical band gap (Eg.opt) of the N typeamorphous semiconductor.

It is preferable that the amorphous semiconductor of the invention shownbythe general formula: a-Si.sub.(1-x-y) C_(x) N_(y), have an opticalbandgap (Eg.opt) of at least about 1.85 eV and a diffusion potential(Vd) of atleast about 1.1 volts. By employing an amorphous semiconductorwhich satisfies this requirement, the heterojunction photovoltaic deviceis allowed to enjoy a great improvement in short-circuit current density(Jsc) and open circuit voltage (Voc). It is further preferred that theelectric conductivity of the amorphous semiconductor of the invention beat least 10⁻⁸ (Ω.cm)⁻¹ at room temperature. When the electricconductivity is less than 10⁻⁸ (Ω.cm.)⁻¹, the fill factor (FF) becomessmall and the conversion efficiency of the obtained photovoltaic deviceis not practical.

The PIN junction photovoltaic device provided by the present inventionwillbe described specifically below. In one typical construction asshown in FIG. 1(a), the device is of the type wherein the light impingeson the P-layer side, and is composed of substrate 1 such as glass,transparent electrode 2, P-type amorphous semiconductor 3, I-typeamorphous silicon 4,N-type amorphous semiconductor 5 and electrode 6.The amorphous silicon carbon nitride semiconductor of the presentinvention is used at least in the layer on which the light impinges, andfor instance, the photovoltaic cell of the invention has theconstruction transparent electrode-P type a-Si.sub.(1-x-y) C_(x) N_(y)-I type a-Si-N type a-Si-electrode, with the transparent electrode sideto be exposed to the incident light. The transparent electrode isdesired to be formed of ITO or SnO₂, preferably the latter. Thetransparent electrode may be previously formed by vacuum deposition on aglass substrate or may be directly formed by vacuum deposition on the Ptype amorphous semiconductor.

The P type a-Si.sub.(1-x-y) C_(x) N_(y) layer on the light impingingside is desired to have a thickness of from about 30 Å to about 300 Å,preferably from 50 Å to 200 Å. Although the thickness of the I type a-Silayer is not specifically limited in the device of the presentinvention, it is generally selected from about 2,500 Å to about 10,000Å. Also, the thickness of the N type a-Si layer is not specificallylimited, but is usually selected from about 150 Å to about 600 Å.Optionally, this N type a-Si layer may be substituted forthe N typea-Si.sub.(1-x-y) C_(x) N_(y) of the present invention.

In another typical construction as shown in FIG. 1(b), the device iscomposed of electrode substrate 7, P-type amorphous semiconductor 8,I-type amorphous silicon 9, N-type amorphous semiconductor 10 andtransparent electrode 11, and the transparent electrode side is exposedtothe light. The photovoltaic device of this type has the construction:transparent electrode-N type a-Si.sub.(1-x-y) C_(x) N_(y) -I type a-Si-Ptype a Si-electrode. The N type a-Si.sub.(1-x-y) C_(x) N_(y) onthe lightimpinging side is desired to have a thickness of from about 30 Å toabout 300 Å, preferably from 50 Å to 200 Å. Although the thickness ofthe I type a-Si is not specifically limited, it is generally selectedfrom about 2,500 Å to about 10,000 Å. The thickness of the P type a-Silayer, which is not particularly limited, is generally selected fromabout 150 Å to about 600 Å. Optionally, this P type a-Si layer may besubstituted for by the P type a-Si.sub.(1-x-y) C_(x) N_(y) of thepresent invention. The material for the transparent electrode and themethod for the vacuum deposition thereof arethe same as described above.

The present inventors have also found that the fill factor and opencircuitvoltage can be greatly improved by providing ITO-SnO₂ layers as atransparent electrode on the light impinging layer of a PIN junctionphotovoltaic device so that the SnO₂ layer has a thickness of about 30 Åto about 500 Å, preferably about 50 Å to 500 Å, and comes into contactwith the light impinging P or N layer. Accordingly, thepresent inventionprovides an amorphous silicon PIN junction photovoltaic device having ahigh conversion efficiency which is improved in that it has a structureof ITO-SnO₂ -P-I-N or ITO-SnO₂ -N-I-P and the thickness of the SnO₂layer is from about 30 Å to about 500 Å.

Typical examples of the basic structure of the photovoltaic deviceaccording to the present invention are shown in FIGS. 4(a) and 4(b).Illustrated in FIG. 4(a) is a photovoltaic device of the type whereinthe light impinges on the P-layer side, and is composed of transparentsubstrate 21 such as glass, ITO film 22, SnO₂ film 23, P-type amorphoussilicon 24, intrinsic amorphous silicon 25, N-type amorphous silicon 26and electrode 27. For instance, the device of this type hastheconstruction: glass-ITO-SnO₂ -P-I-N-Al. Illustrated in FIG. 4(b) is aphotovoltaic device of the type wherein the light impinges on theN-layer side, and is composed of ITO film 28, SnO₂ film 29, N-typeamorphous silicon 30, intrinsic amorphous silicon 31, P-type amorphoussilicon 32 and electrode 33 such as metal. For instance, the device ofthis type has the construction: stainless steel-P-I-N-SnO₂ -ITO. Otherconstructions may be formed, for instance, by interposing a thininsulation layer or a thin metal layer between the P-layer or N-layerand the transparent electrode layer. Any constructions are adoptable aslong as the transparent electrode of the ITO-SnO.sub. 2 two layerstructure in which the SnO₂ layer has a thickness of about 30 Å andabout 500 Å, is provided on the P-layer or N-layer to be exposed to thelight toform an ITO-SnO₂ -P-I-N or ITO-SnO₂ -N-I-P structure. Theabove-mentioned specific transparent electrode is applicable to the a-SiPIN homojunction and heterojunction photovoltaic devices.

The ITO film in the present invention is formed by subjecting In₂ O₃containing 3 to 15% by weight of SnO₂ to an electron beam deposition orsputtering method. The SnO₂ film of the present invention is usuallydoped with a slight amount of Sb, and is formed by anelectron beamdeposition, sputtering or chemical vapor deposition method. In the caseof depositing on a transparent substrate 1 such as glass as shown inFIG. 4(a), ITO is deposited on the substrate and SnO₂ is then depositedthereon as a film having a thickness of about 30 to about 500 Å.Although the thickness of the ITO film is not particularly limited, itis usually selected from 600 Å to 4,000 Å, especially from 600 Å to2,000 Å. In the case of employing a metal electrode 33 as shown in FIG.4(b), after forming amorphous silicon semiconductors 32, 31 and 30 onthe metal electrode 33, the SnO₂ film 29 having a thickness of 30 to 500Å and the ITO film are formed thereon in that order.

Further improved results are obtained by fabricating a PINheterojunction photovoltaic device employing the transparent electrodeof the ITO-SnO₂ two layer structure. In that case, an amorphous siliconsemiconductor of the general formula: a-Si.sub.(1-x) C_(x) ora-Si.sub.(1-y) N_(y), is employed in at least the layer which comesintocontact with the SnO₂ layer, as well as the before-mentioneda-Si.sub.(1-x-y) C_(x) N_(y) semiconductor of the present invention. Theamorphous silicon carbide is formed by the glow discharge decompositionof a mixed gas containing a silicon compound and a carbon compound suchas hydrocarbons, and the amorphous silicon nitride is formedby the glowdischarge decomposition of a mixed gas containing a silicon compound anda nitrogen compound such as ammonia. They are doped with a P-type orN-type impurity such as phosphine or diborane.

The present invention is more specifically described and explained bymeansof the following Examples.

It is to be understood that the present invention is not limited to theExamples and various changes and modifications may be made in theinvention without departing from the spirit and scope thereof.

EXAMPLE 1

Glow discharge decomposition was carried out at a radio-frequency of13.56 MHz in a quartz tube having an inner diameter of 11 cm. I-typea-Si was obtained by subjecting silane diluted with hydrogen to the glowdischarge decomposition at 2 to 10 Torrs. N-type a-Si was obtainedsimilarly by subjecting silane diluted with hydrogen and phosphine (PH₃)(PH₃/SiH₄ =0.5 mole %) to the glow discharge decomposition. P-typea-Si.sub.(1-x-y) C_(x) N_(y) was obtained similarly by subjecting silanediluted with hydrogen, methane (CH₄), ammonia (NH₃) and diborane (B₂ H₆)[B/(Si+C+N)=0.50 atom %] to the glow discharge decomposition. In thatcase, the gas composition involved in the glow discharge decompositionwas adjusted so that the atomic fraction (x+y) in the formula:a-Si.sub.(1-x-y) C_(x) N_(y), would fall in the range of from 0.80 to0.05.

A solar cell was constructed by successively depositing the P-typea-Si.sub.(1-x-y) C_(x) N_(y), the I-type a-Si and the N-type a-Si in theorder mentioned on the SnO₂ film of 25 Ω/□ and finally vacuum depositingaluminum of 3.3 mm². During the glow discharge, the temperature of thesubstrate was kept at 250° C. Thethickness of the I-layer was 5,000 Å,the thickness of the N-layer was 500 Å and the thickness of the P-typea-Si.sub.(1-x-y) C_(x) N_(y)layer was 135 Å. This solar cell was testedfor cell properties by use of a AM-1 solar simulator made by UshioElectric Industry Co., Ltd. under a solar illumination power of 100mW/cm².

The characteristics of the obtained solar cells having a construction ofglass/SnO₂ /P-type a-Si.sub.(1-x-y) C_(x) N_(y) :H/I-type a-Si:H/N-typea-Si:H/Al are shown in Table 1 with respect to various compositions ofthe P-type a-Si.sub.(1-x-y) C_(x) N_(y).

                                      TABLE 1                                     __________________________________________________________________________    Composition of a-Si.sub.(1-x-y) C.sub.x N.sub.y                               Atomic fraction x 0    0.05 0.10 0.10 0.20 0.20 0.30 0.50 0.10                Atomic fraction y 0    0.05 0.10 0.20 0.10 0.20 0.30 0.10 0.50                Optical energy gap Eg 19 0 opt (eV)                                                             1.76 1.89 1.98 2.10 2.08 2.15 2.25 2.23 2.30                Electric conductivity (Ω · cm).sup.-1                                            5 × 10.sup.-7                                                                9 × 10.sup.-7                                                                6 × 10.sup.-7                                                                3 × 10.sup.-7                                                                3 × 10.sup.-7                                                                1 × 10.sup.-7                                                                9 × 10.sup.-8                                                                9 × 10.sup.-8                                                                5                                                                             ×10.sup.-8                                                              7                   Ef - Ev (eV)      0.6  0.57 0.50 0.54 0.55 0.60 0.63 0.62 0.70                Diffusion potential Vd (V)                                                                      0.96 1.12 1.28 1.36 1.33 1.35 1.42 1.41 1.40                Solar cell characteristics                                                    Short-circuit current                                                         density Jsc (mA/cm.sup.2)                                                                       10.0 11.3 11.7 12.3 12.2 12.6 13.5 13.4 13.6                Open circuit voltage                                                          Voc (volt)        0.75 0.84 0.90 0.91 0.91 0.92 0.95 0.94 0.93                Curve fill factor FF (%)                                                                        0.61 0.69 0.67 0.65 0.65 0.64 0.59 0.60 0.53                Conversion efficiency η (%)                                                                 4.6  6.5  7.1  7.3  7.2  7.4  7.6  7.6  6.7                 __________________________________________________________________________    (Note)                                                                        I-type aSi:H Eg · opt = 1.8 eV                                       Ec - Ef = 0.85 eV                                                             N-type aSi:H Eg · opt = 1.8 eV                                       Ec - Ef = 0.2 eV                                                          

It is seen in Table 1 that the conversion efficiency which is 4.6% forthe P-layer made from silane alone (i.e. P-type a-Si:H) is increased to6.5% even when the values of "x" and "y" in the amorphous semiconductora-Si.sub.(1-x-y) C_(x) N_(y) :H of the present invention are 0.05 and0.05 respectively, and is increased to 7.3% when "x" is 0.10 and "y" is0.20, and is increased to 7.6% when "x" is 0.3 and "y" is 0.3, thusindicating that the amorphous silicon carbon nitride of the inventionusedas P-layer of PIN heterojunction solar cells permits a markedimprovement in conversion efficiency as compared with the use of P-typea-Si:H. It should be noted here that while the increase in theshort-circuit current density (Jsc) is naturally expected from the factthat the optical energy gap of the a-Si.sub.(1-x-y) C_(x) N_(y) isgreater than that of the a-Si, the increase in the open circuit voltage(Voc) is quite beyond expectation. The conspicuous improvement in theconversion efficiency is attained by the improvement in these twofactors.

Substantially the same results were obtained when SiF₄, CH₄ and NH₃ wereused instead.

The optical energy gap (Eg.opt) of the a-Si.sub.(1-x-y) C_(x) N_(y) ishigher than that of the a-Si as shown in Table 1, and accordingly theuse of the amorphous semiconductor of the invention as window materialsis naturally expected to bring about an improvement in short-circuitcurrent density (Jsc). Moreover, it brings about an unexpectedconspicuous improvement in open circuit voltage (Voc). The reasontherefor evidently exists in the relation between the diffusionpotential (Vd) and the open circuit voltage (Voc), which plots in astraight line, as shown in FIG. 3.This means that the value of Voclinearly increases in proportion to the increase of Vd. This factindicates that the diffusion potential is enhanced and Voc isproportionately improved by using an amorphous semiconductor having alarge optical energy gap as a material for the window of the PINjunction photovoltaic device.

As described above, when the amorphous semiconductor having an Eg.opt ofatleast about 1.85 eV and a PIN junction diffusion potential (Vd) of atleast1.1 volts is employed as a window material of a heterojunctionphotovoltaicdevice, marked improvements are obtained in not only Jsc,but also Voc. Thesame result is obtained also in the case where theN-type a-Si.sub.(1-x-y) C_(x) N_(y) is used as the N-layer of aphotovoltaic device of the type wherein the light impinges on theN-layer side, as shown in FIG. 1(b).

EXAMPLE 2

PIN junction solar cells were prepared by employing the following 7kinds of substrates.

(1) Glass/ITO (1000 Å, 15 Ω/□)

(2) Glass/SnO₂ (2500 Å, 25 Ω/□)

(3) Glass/ITO (1000 Å)/SnO₂ (30 Å) (15 Ω/□)

(4) Glass/ITO (1000 Å)/SnO₂ (50 Å) (15 Ω/□)

(5) Glass/ITO (1000 Å)/SnO₂ (100 Å) (15 Ω/□)

(6) Glass/ITO (1000 Å)/SnO₂ (300 Å) (15 Ω/□)

(7) Glass/ITO (1000 Å)/SnO₂ (500 Å) (15 Ω/□)

The ITO and SnO₂ films were formed by a sputtering method.

Glow discharge decomposition was carried out at a radio-frequency of13.56 MHz in a quartz tube having an inner diameter of 11 cm. Thetemperature ofthe substrate was maintained at 250° C. P, I and N layerswere formed on the substrate by depositing amorphous silicon layers inthe order of P-type, I-type and N-type under the following conditions,and finally an aluminum electrode of 1 cm.² was formed on the N-layer byvacuum deposition to give PIN junction solar cells.

[Conditions of the preparation of P, I and N layers]

(a) Intrinsic amorphous silicon (I-type a-Si:H) SiH₄ /H₂, 3 Torr,5000 Åin thickness

(b) N-type amorphous silicon (N-type a-Si:H) PH₃ /SiH₄ =0.5%, 3 Torr,500 Å in thickness

(c) P-type amorphous silicon (P-type a-Si:H) B₂ H₆ /SiH₄ =0.2%, 3 Torr,100 Å in thickness

(d) P-type amorphous silicon carbide (P-type a-SiC:H) B₂ H₆ /(SiH₄+CH₄)=0.1%, SiH₄ /CH₄ =3/7 3 Torr, 100 Å inthickness

(e) P-type amorphous silicon nitride (P-type a-SiN:H) B₂ H₆ /(SiH₄+NH₃)=0.1%, SiH₄ /NH₃ =1/1 3 Torr, 100 Å inthickness

The change in the conversion efficiency of the obtained PIN junctionsolar cells on the basis of the difference in the substrate was observedby measuring the solar cell characteristics with the AM-1 solarsimulator (100 mW/cm.²).

The results for the P-type a-Si:H/I-type a-Si:H/N-type a-Si:H solarcell, the P-type a-SiC:H/I-type a-Si:H/N-type a-Si:H solar cell and theP-type a-SiN:H/I-type a-Si:H/N-type a-Si:H solar cell are shown inTables 2-1, 2-2 and 2-3, respectively, in which "Jsc" shows theshort-circuit current density, "Voc" shows the open circuit voltage,"FF" shows the fill factor and "η" shows the conversion efficiency.

                  TABLE 2-1                                                       ______________________________________                                        (P, a-Si:H/I, a-Si:H/N, a-Si:H)                                               Substrate Jsc       Voc        FF   n                                         No.       (mA/cm.sup.2)                                                                           (V)        (%)  (%)                                       ______________________________________                                        1         11.0      0.75       0.68 5.6                                       2         10.5      0.80       0.57 4.8                                       5         11.0      0.80       0.67 5.9                                       ______________________________________                                    

                  TABLE 2-2                                                       ______________________________________                                        (P, a-SiC:H/I, a-Si:H/N, a-Si:H)                                              Substrate Jsc       Voc        FF   n                                         No.       (mA/cm.sup.2)                                                                           (V)        (%)  (%)                                       ______________________________________                                        1         13.0      0.82       0.66 7.0                                       2         12.1      0.91       0.56 6.2                                       3         13.0      0.85       0.66 7.3                                       4         13.0      0.91       0.66 7.8                                       5         12.9      0.91       0.67 7.9                                       6         12.7      0.91       0.67 7.7                                       7         12.6      0.91       0.65 7.5                                       ______________________________________                                    

                  TABLE 2-3                                                       ______________________________________                                        (P, a-SiN:H/I, a-Si:H/N, a-Si:H)                                              Substrate Jsc       Voc        FF   n                                         No.       (mA/cm.sup.2)                                                                           (V)        (%)  (%)                                       ______________________________________                                        1         12.8      0.81       0.66 6.8                                       2         12.4      0.91       0.57 6.4                                       5         12.8      0.91       0.65 7.6                                       ______________________________________                                    

It is seen in Table 2-1 that even in the case where hydrogenatedamorphous silicon (a-Si:H) is used in the P-layer of a PIN junctionsolar cell, the conversion efficiency is improved by the use of the No.5 substrate, i.e. glass/ITO (1000 Å)/SnO₂ (100 Å) (15 Ω/□). Further, itis observed in Tables 2-1 and 2-2 that the effect on the improvement ofconversion efficiency is particularly marked when the substrate of theglass/ITO/SnO₂ type is directly brought into contactwith the P-typea-SiC:H or P-type a-SiN:H. Also, it is seen from Table 2-2 that even ifthe substrate is of the glass/ITO/SnO₂ type, the substrate having anSnO₂ layer of not less than 50 Å in thickness(substrate Nos. 4, 5 and 6)is more preferable than that having a SnO₂layer of less than 50 Å inthickness (substrate No. 3). Further, it is found that when thesubstrate having a SnO₂ layer of 500 Å in thickness is employed, theconversion efficiency is decreased somewhat.

EXAMPLE 3

Glow discharge decomposition was carried out in the same manner as inExample 2 by employing a stainless steel plate as a metal electrode.Inverted PIN junction solar cells were prepared by depositing amorphoussilicon semiconductors on the metal substrate in the order of P, I and Nlayers under the following conditions and then providing the followingtransparent electrode on the N-layer by means of electron beamdeposition.

[Transparent electrode]

(1) ITO (1000 Å, 15 Ω/□)

(2) SnO₂ (2500 Å, 25 Ω/□)

(3) SnO₂ (100 Å)+ITO (1000 Å) (15 Ω/□)

In the case of the transparent electrode (3), it was formed such thatthe SnO₂ layer came into contact with the N-layer.

[Conditions of the preparation of P, I and N layers]

(a) Intrinsic amorphous silicon (I-type a-Si:H) 4000 Å in thickness

(b) P-type amorphous silicon (P-type a-Si:H) B₂ H₆ /SiH₄ =1.0%, 300 Å inthickness

(c) N-type amorphous silicon (N-type a-Si-H) PH₃ /SiH₄ =0.5%, 100 Å inthickness

(d) N-type amorphous silicon carbide (N-type a-SiC:H) PH₃ /(SiH₄+CH₄)=0.5%, SiH₄ /CH₄ =1/1, 100 Å in thickness

(e) N-type amorphous silicon nitride (N-type a-SiN:H) PH₃ /(SiH₄+NH₃)=0.5%, SiH₄ /NH₃ =1/1, 100 Å in thickness

The change in the conversion efficiency of the obtained inverted PINjunction solar cells on the basis of the difference in the transparentelectrode was observed by measuring the solar cell characteristics inthe same manner as in Example 2.

The results for the P-type a-Si:H/I-type a-Si:H/N-type a-Si:H solarcell, the P-type a-Si:H/I-type a-Si:H/N-type a-SiC:H solar cell and theP-type a-Si:H/I-type a-Si:H/N-type a-SiN:H solar cell are shown inTables 3-1, 3-2, 3-3, respectively.

                  TABLE 3-1                                                       ______________________________________                                        (P, a-Si:H/I, a-Si:H/N, a-Si:H)                                               Transparent Jsc       Voc       FF   n                                        electrode   (mA/cm.sup.2)                                                                           (V)       (%)  (%)                                      ______________________________________                                        1           10.5      0.80      0.65 5.5                                      2           10.3      0.87      0.58 5.2                                      3           10.5      0.87      0.66 6.0                                      ______________________________________                                    

                  TABLE 3-2                                                       ______________________________________                                        (P, a-Si:H/I, a-Si:H/N, a-SiC:H)                                              Transparent Jsc       Voc       FF   n                                        electrode   (mA/cm.sup.2)                                                                           (V)       (%)  (%)                                      ______________________________________                                        1           11.7      0.85      0.65 6.5                                      2           11.4      0.93      0.59 6.3                                      3           11.8      0.94      0.67 7.4                                      ______________________________________                                    

                  TABLE 3-3                                                       ______________________________________                                        (P, a-Si:H/I, a-Si:H/N, a-SiN-H)                                              Transparent Jsc       Voc       FF   n                                        electrode   (mA/cm.sup.2)                                                                           (V)       (%)  (%)                                      ______________________________________                                        1           11.5      0.84      0.64 6.2                                      2           11.1      0.89      0.57 5.6                                      3           11.6      0.90      0.66 6.9                                      ______________________________________                                    

It is found from Tables 3-1, 3-2 and 3-3 that the conversion efficiencyis remarkably improved also in the fabrication of inverted PIN junctionsolarcells whose N-layer side is exposed to the light, by the use of theITO/SnO₂ transparent electrode which is provided by forming the SnO₂layer on the N-layer and then forming the ITO layer on the SnO₂ layer.

In addition to the ingredients used in the Examples, other ingredientscan be used in the Examples as set forth in the specification to obtainsubstantially the same results.

What we claim is:
 1. In an amorphous silicon derivative PIN junctionphotovoltaic device, the improvement comprising a two layer transparentelectrode consisting of ITO and SnO₂ layers that are provided on the Por N layer located on the light impinging side so that the SnO₂ layer isin contact with said P or N layer, said SnO₂ layer having a thickness ofabout 30 to about 500 angstroms.
 2. The photovoltaic device of claim 1,wherein said SnO₂ layer has a thickness of about 50 to about 500angstroms.
 3. The photovoltaic device of claim 1, wherein the amorphoussemiconductor of said P or N layer which contacts the SnO₂ layer is amember selected from the group consisting of an amorphous semiconductorof the general formula: a-Si.sub.(1-x) C_(x), an amorphous semiconductorof the general formula: a-Si.sub.(1-y) N_(y), and an amorphoussemiconductor of the general formula: a-Si.sub.(1-x-y) C_(x) N_(y). 4.The photovoltaic device of claim 1, wherein the amorphous semiconductorof said P or N layer which contacts the SnO₂ layer is an amorphoussemiconductor of the general formula: a-Si.sub.(1-x-y) C_(x) N_(y).